1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries that contain a lithium cobalt oxide as the positive electrode active material.
2. Description of Related Art
A high energy density battery that has drawn attention in recent years is a non-aqueous electrolyte secondary battery that employs a negative electrode active material composed of a carbon material, metallic lithium, or an alloy capable of absorbing and desorbing lithium ions, and a positive electrode active material made of a lithium-transition metal composite oxide represented by the chemical formula LiMO2 (where M is a transition metal)
A representative example of the lithium-containing transition metal complex oxide is lithium cobalt oxide (LiCoO2), which has been already put to practical use as the positive active material for non-aqueous electrolyte secondary batteries. For non-aqueous electrolyte secondary batteries using a lithium transition metal oxide, such as lithium cobalt oxide, as the positive active material and a carbon material, such as graphite, as the negative active material, an end-of-charge voltage is generally prescribed at 4.1-4.2 V. In this case, the active material of the positive electrode utilizes only 50-60% of its theoretical capacity. Accordingly, if the end-of-charge voltage is increased to a higher value, a capacity (utilization factor) of the positive electrode can be improved to increase the battery capacity and energy density.
However, when the end-of-charge voltage of the battery is increased to a higher value, deterioration in the structure of LiCoO2 and decomposition of the electrolyte solution at the positive electrode surface tend to easily occur. In particular, when the battery is stored in a charged state at high temperature, a problem arises that the charge-discharge performance of the battery degrades due to a battery thickness increase originating from a gas generated by the reaction between the positive electrode and the electrolyte solution, a resistance increase resulting from reaction products, and disintegration of the positive electrode material.
Japanese Published Unexamined Patent Application No. 7-272756 proposes that, in a non-aqueous electrolyte secondary battery using a lithium cobalt oxide as a positive electrode active material and a graphite material as a negative electrode active material, the electrolyte solution contain a compound having an ester group such as an alkyl ester therein, in order to improve the high-temperature storage characteristics in a charged state.
Even with the addition of the compound having an ester group to the electrolyte solution, however, the battery swelling and deterioration in battery performance that originate from high-temperature storage in a charged state are considerable in the case that the charge voltage of the battery is increased to a higher value than conventionally used 4.2 V (that is, in the case that the charge potential of the positive electrode is increased to a higher value than 4.3 V versus a lithium reference [hereinafter expressed as “4.3 V (vs. Li/Li+)”]). Thus, the foregoing technique has been unable to achieve sufficient improvements.